In a pressurized water reactor nuclear power plant, a reactor coolant pump is usually called a main pump. A liquid static pressure shaft sealing type nuclear main pump is a single-stage, single-suction upstanding mixed-flow pump driven by a three-phase induction type motor. FIG. 1 is a schematic view of a main pump of a typical second-generation and its improved nuclear power plant, wherein the main pump consists of, from up to down, a motor, a shaft sealing assembly and a water power member. A shaft (briefly called pump shaft) of the reactor coolant pump runs through a center of the whole pump. The reactor coolant is pumped in a fluid form by an impeller mounted at a lower end of the pump shaft, the coolant is sucked in through a bottom of the pump housing, and flows upwards through the impeller, and then is discharged through a guide vane and an outlet pipe on a lateral side of the pump housing.
FIG. 2 and FIG. 3 are respectively a schematic view of a static pressure shaft sealing assembly and a first sealing assembly of the main pump shown in FIG. 1. The shaft sealing assembly employs a three-stage shaft sealing, respectively a first sealing assembly (also called No. 1 sealing), a second sealing assembly (also called No. 2 sealing) and a third sealing assembly (also called No. 3 sealing) from bottom to top. Under normal operation conditions, the cooling of the first sealing assembly is guaranteed by infill water provided by a chemical and volume control system (shortly called RCV). Under station blackout (shortly called SBO) conditions, the RCV system loses its function and cannot provide normal cooling for the shaft sealing assembly in the main pump. Meanwhile, an equipment cooling water system (shortly RRI) also loses its function and cannot provide standby cooling for the shaft sealing assembly in the main pump. At this time, high-temperature fluid of a loop quickly threatens the shaft sealing assembly of the main pump, and its thermal stress might cause the loss of the main pump shaft sealing function, thereby damaging a loop pressure boundary.
As shown in FIG. 4, in the prior art, emergent seal injection is usually used to solve the integrity issue of main pump shaft sealing under the SBO condition. After occurrence of SBO, a hydraulic test pump diesel generator set supplies power to a loop hydraulic test pump RIS011P0. In an emergent case, after an activation instruction is received, the hydraulic test pump diesel generator set is put into use within two minutes to ensure that the hydraulic test pump emergently injects water to the main pump shaft sealing assembly to maintain the cooling and lubrication at the location of the first sealing assembly and meanwhile limit the high-temperature high-pressure reactor coolant downstream the first sealing assembly to ensure that the temperature at the location of the first sealing assembly is within a scope as required by the operation thereof and prevent loss of coolant accident (abbreviated as LOCA) of the main pump shaft sealing assembly so as to ensure integrity of a loop pressure boundary.
However, the present-day second-generation and improved nuclear power plants are mostly of a double-reactor configuration. Two sets of units share one hydraulic test pump. The design only considers blackout accident of a single set of unit, a nominal flow of the hydraulic test pump is 6 m3/h and the flow can only meet the requirement for amount of injection of shaft sealing water of one set of unit, namely, three sets of main pumps. Upon occurrence of SBO, emergent shaft seal injection of main pumps of the other set of unit cannot be ensured, seal LOCA will be caused. After all water replenishing means are lost, leakage of one loop cannot be replenished, water amount cannot be ensured so that the reactor core gradually gets exposed and finally melts.